IDENTIFICATION OF THE MIDBRAIN LOCOMOTOR REGION AND ITS RELATION TO DESCENDING LOCOMOTOR PATHWAYS IN THE ATLANTIC STINGRAY, DASYATIS-SABINA

The midbrain locomotor region (MLR) in the Atlantic stingray, Dasyatis sabina, was identified and characterized. Stimulation (50-100-mu-A, 60 Hz) of the midbrain in decerebrated, paralyzed animals was used to elicit locomotion monitored as alternating activity in nerves innervating an antagonist pair of elevator and depressor muscles. Effective sites for evoking locomotion in the midbrain included parts of several nuclei: the caudal portion of the interstitial nucleus of the medial longitudinal fasciculus and the caudomedial parts of the cuneiform and subcuneiform nuclei. This region did not include the red nucleus, any parts of the optic tectum or the medial or lateral mesencephalic nuclei. Electrical stimulation in the MLR evokes locomotion in either the ipsi- or contralateral pectoral fin, whereas stimulation in the medullary reticular formation evokes locomotion only in the contralateral fin. Lesion experiments were performed to identify the location of descending pathways from the midbrain to the medullary reticular formation. To abolish locomotion evoked by electrical stimulation in the MLR, the medial reticular formation in the rostral medulla had to be lesioned bilaterally, or the ipsilateral medial medullary reticular formation and fibers projecting from the MLR to the contralateral midbrain had to be disrupted. Injections of HRP into the magnocellular/gigantocellular reticular formation confirmed that this area received bilateral projections from the MLR. The MLR of the Atlantic stingray appears to be similar to the lateral component of the mammalian MLR and to the MLR in other non-mammalian vertebrates.